This particular form of sexual selection relies on females being able to distinguish between differently coloured males. But as pollution clouds the waters of Africa's great lakes, cichlids are losing this ability. In the murky waters, hybridisation is becoming increasingly common, and because cichlid species are evolutionarily close, they often produce viable hybrid offspring. Surprisingly, some biologists now think that hybridisation might actually be a creative process, churning out new species, and it has probably happened naturally in Lake Victoria many times in the past. I am beginning to suspect that hybridisation may be a significant factor in some of the evolutionary explosions we call adaptive radiations.

In theory, we can test whether species are the product of parallel evolution, sexual selection or hybridisation by looking for "speciation genes" - genes that are responsible for preventing interbreeding. As more and more genomes are sequenced, biologists eagerly anticipate finding such genes. Also, there is a big push to look at differences in the way genes are expressed. These are nice ideas, but I don't think we know enough about which genes are involved in speciation to give us a realistic chance of finding them by such methods. We would do better to use careful Mendel-style crossing experiments to find out if speciation can really be caused by a single gene or a pair of genes, like a male courtship signal and a female preference for that signal. I think most people would bet against this being common. But then, most studies are carried out on relatively old pairs of species that are likely to have evolved lots of other differences following speciation. So, we need to look at species that have recently diverged and can still be crossed in the lab to give fertile hybrids. My old friends the cichlids look like the perfect candidates.

George Turner is an evolutionary biologist and behavioural ecologist at the University of Hull, UK.

4. Is evolution predictable?

The late Stephen Jay Gould once famously suggested a thought experiment in which the tape of life is rewound and replayed. Would the repetition bear any resemblance to the original? Gould's answer was that it would not: each play of the tape would produce a different outcome. His answer stemmed from the fact that evolution proceeds from a continuous interplay of both random and selective forces. The existence of a random element (mutation, recombination and migration) and a stochastic component (daily chance events that determine the survival of individuals and the probability of finding a mate) suggest that evolution cannot be repeatable, predictable or even follow rules.

Yet, as Darwin so clearly saw, working hand in hand with contingency is natural selection, a most potent force that systematically sorts among variant types, favouring characteristics in organisms that give them a better chance of surviving. Indeed, Darwin's theory of natural selection makes a prediction - that organisms will adapt to their environment.

Importantly, though, any predictions based on Darwin's theory will be probabilistic: they require us to know the odds against particular events happening. The trouble is, we rarely do. But all is not lost. Although today's evolutionary biologists do not anticipate "laws" analogous to those in the physical sciences, as Darwin and other 19th-century biologists did, there is mounting evidence for the existence of certain fundamental rules of evolution. Our growing understanding of the mechanism of evolutionary change is providing tantalising hints that certain outcomes may be more likely than others.

The evidence stems from research on topics as diverse as language and learning theory, evolutionary and developmental genetics, biochemical systems theory and metabolic network analysis. What these all have in common is their focus on establishing the basic design principles of complex systems. It is becoming clear that such systems are often assembled from combinations of a few simple modules. The loose linkage that typically exists between modules allows a huge number of possible combinations. It also ensures that different combinations of modules have a high probability of generating biologically viable scenarios. In evolutionary terms, this suggests that even though there may be a limited number of successful solutions to a particular evolutionary challenge, there may be many ways of achieving the same end.

Fortunately, there is a way of testing some of these ideas. My colleagues and I study populations of the bacterium Pseudomonas fluorescens that rapidly diversify by mutation and selection into distinct types or "morphs" when you grow them in test tubes of nutrient broth. These experiments in test-tube evolution allow us to replay life's tape, albeit on a small scale, as often as we like.

We have found that when we seed our mini-worlds with genetically identical microbes and the population size is large (around a billion cells per millilitre), each "replay" results in highly similar patterns of evolutionary change. After just one week, P. fluorescens evolves into two new morphs that we call "wrinkly" and "fuzzy" spreaders. But this doesn't happen if we limit the mutation supply rate, reducing it by more than two orders of magnitude. Evolution only repeats itself if certain phenotypic innovations have a high probability of arising and are strongly favoured by selection.

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How did life emerge on the primordial Earth, and how has it developed since? (Image: Darren Greenwood / Design Pics Inc. / Rex Features)